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Alt bearing adjument on Zhumell Dobson

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9 replies to this topic

#1 Mainecanoe

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Posted 07 December 2017 - 05:39 AM

How can I tighten the alt bearings on zhumell dobson? When I acquire my target, there remains play in the alt even when I've tighten the knobs.

#2 wrnchhead

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Posted 07 December 2017 - 09:08 AM

Ok the bearings aren't intended to lock the alt movement. Balance plus some pre-load on the bearings should keep it steady. If you tighten them too much, they will eventually be damaged. 

All of the load on the bearings (squeezing effect) are done with the knobs. 

 

I'm assuming you have the newer designed GSO dob bearing design. 

 

Like this

post-11853-0-65312800-1339793391.jpg



#3 Muddman97

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Posted 07 December 2017 - 09:34 AM

On my 10 inch, I had to move my bearing up the slide about half way up to the "1" on the scale (see Eric's pic) to get mine to properly balance with the Telrad, ES82 eyepiece, RA finder, and light shield attached.

 

Then I could run with just light tension on the knobs rather than having to really crank them down to maintain position, which you don't want to do for the same reason Eric mentioned.  



#4 kfiscus

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Posted 07 December 2017 - 10:03 AM

Good advice above.  Here's how to adjust your bearing position.  Load your OTA with a medium to heavy eyepiece and attach your finderscope.  Carefully balance the tube on the floor, using a broom handle under the OTA for the pivot.  (Be very gentle to avoid denting the tube.  It's seam is on the bottom of the OTA and is its strongest part.)  Loosen the pair of bolts on each altitude hub and slide them to matching marks at your balance point.  Re-tighten the paired adjustment bolts.  Check for balance.



#5 Mainecanoe

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Posted 07 December 2017 - 10:49 AM

Thanks.

Those are the knobs I have. From what I'm experiencing, It's not a balancing issue.

I've been cranking down on the black knobs when I store it. When tight, there is still play. It's not ruining my observing, but is annoying as you can imagine. Has to be a fix out there!

#6 Cames

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Posted 07 December 2017 - 11:52 AM

Thanks.

Those are the knobs I have. From what I'm experiencing, It's not a balancing issue.

I've been cranking down on the black knobs when I store it. When tight, there is still play. It's not ruining my observing, but is annoying as you can imagine. Has to be a fix out there!

 

Have you inspected the slot in the base that the altitude trunnion settles into?  Look closely at the space surrounding the aluminum block insert as you gently rock the OTA up and down. If the aluminum block looks like it is rocking or rotating a little bit, you could try wedging some cardboard into the gap and see if it makes a difference. If it helps, you have diagnosed where the problem is.  Knowing the source of your problem is the first step in solving it.  Use magnification if you need it.

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C

 

Hint: credit cards make nice stiff shims


Edited by Cames, 07 December 2017 - 01:24 PM.


#7 bleep

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Posted 07 December 2017 - 09:32 PM

 

Thanks.

Those are the knobs I have. From what I'm experiencing, It's not a balancing issue.

I've been cranking down on the black knobs when I store it. When tight, there is still play. It's not ruining my observing, but is annoying as you can imagine. Has to be a fix out there!

 

Have you inspected the slot in the base that the altitude trunnion settles into?  Look closely at the space surrounding the aluminum block insert as you gently rock the OTA up and down. If the aluminum block looks like it is rocking or rotating a little bit, you could try wedging some cardboard into the gap and see if it makes a difference. If it helps, you have diagnosed where the problem is.  Knowing the source of your problem is the first step in solving it.  Use magnification if you need it.

-----------

C

 

Hint: credit cards make nice stiff shims

 

I had the same issue with my AD10 where there was just a little play in the base where the bearings sit. One side of the trunnion had felt but the other was bare. So I simply added a piece of moleskin to the other side for a tighter fit and that helped that slight rocking problem I was having. I imagine if you can't find a piece of moleskin maybe a piece of Velcro (fuzzy side) with adhesive backing should work the same. 



#8 Mainecanoe

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Posted 08 December 2017 - 05:44 AM

Thanks. I'll check and let all know my findings. Happy Friday!
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#9 wrnchhead

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Posted 08 December 2017 - 09:07 AM

Interesting, I never considered it could be the bearing to base mating area. Mine has adhesive felt that's so tight, it can be a struggle to insert/remove my OTA. I haven't modified it because I figure over time it will get looser every time I assemble/disassemble. 



#10 Cames

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Posted 08 December 2017 - 09:58 AM

Interesting, I never considered it could be the bearing to base mating area. Mine has adhesive felt that's so tight, it can be a struggle to insert/remove my OTA. I haven't modified it because I figure over time it will get looser every time I assemble/disassemble. 

You make an excellent point about inserting and removing the OTA.

 

The OTA is heavy enough as it is. The last thing one needs when it comes time for lifting the telescope tube off the base is the added force needed to overcome friction fit between the two parts.

 

As the OP has lamented, if an interstitial gap exists there, slewing the altitude to point to and track objects becomes frustratingly imprecise.

 

I wish that a latch could be designed into the base.  The latch mechanism could be opened wide when inserting and removing the OTA; and then closed to clamp the parts tight together to facilitate accurate slewing of the telescope.

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C


Edited by Cames, 08 December 2017 - 10:34 AM.



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